As is well known, the viscosity of lubricating oils varies with temperature, and since lubricating oils generally incur a relatively broad temperature range during use, it is important that the oil not be too viscous (thick) at low temperatures nor too fluid (thin) at high temperatures. In this regard, variation in the viscosity-temperature relationship of an oil is indicated by the so-called viscosity index (VI). The higher the viscosity index, the less the change in viscosity with temperature. In general, the viscosity index is a function of the oil viscosity at a given lower temperature and a given higher temperature. In the United States, the given lower temperature and the given higher temperature for cars have varied over the years but are fixed at any given time in an ASTM test procedure (ASTM D 2270). Currently the lower temperature specified in the test is 40.degree. C. and the higher temperature in the test is 100.degree. C.
Heretofore, several methods have been proposed for improving the rheological properties of lubricating oil compositions. Frequently these methods involve the use of one or more polymeric additives. Methods of this type wherein the polymeric additive is a linear or branched chain polymer, are taught, for example in U.S. Pat. Nos. 3,554,911; 3,668,125; 3,772,196; 3,775,329 and 3,835,053. The polymeric additives taught in this series of U.S. patents are, generally, hydrogenated, linear or substantially linear polymers of one or more conjugated dienes which may, optionally, also contain monomeric units of one or more monoalkenyl aromatic hydrocarbons. Polymers of the type disclosed in this series of patents are typically prepared via the anionic solution polymerization of the monomers followed by hydrogenation. A selectively hydrogenated, linear block copolymer comprising a single styrene polymer block and a single selectively hydrogenated isoprene polymer block which block copolymer is within the scope of U.S. Pat. No. 3,772,196 is available commercially and is commonly used a VI improver.
Methods of this type wherein the polymeric additive is a star-shaped polymer are taught, for example, in U.S. Pat. Nos. 4,077,893; 4,116,917; 4,141,847; 4,156,673; and 4,427,834. This polymeric additives taught in this series of patents are, generally, star-shaped (radial) polymers wherein all, or at least most, of the polymer arms will be either homopolymers or copolymers of conjugated dienes or copolymers of one or more conjugated dienes and one or more monoalkenyl aromatic hydrocarbons. In certain of these polymeric additives, however, one or more arms will either be modified or different. Polymers wherein all of the arms are either hydrogenated homopolymers or copolymers of one or more conjugated diolefins and one or more monoalkenyl aromatic hydrocarbons or a mixture of such arms are described in U.S. Pat. Nos. 4,116,917 and 4,156,673. Star polymers of the type disclosed in U.S. Pat. Nos. 4,116,197 and 4,156,673 which are modified to incorporate functional groups imparting dispersant characteristics are taught in U.S. Pat. Nos. 4,077,893, 4,141,847 and 4,427,834. The polymeric additives disclosed in U.S. Pat. No. 4,077,893 are hydrogenated star-shaped polymers which are reacted first with an unsaturated carboxylic acid or derivative thereof and then with an alkane polyol. The polymeric additives disclosed in U.S. Pat. No. 4,141,847 are hydrogenated star-shaped polymers which are reacted first with an unsaturated carboxylic acid or derivative thereof and then with an amine. The polymeric additive taught in U.S. Pat. No. 4,427,834 is one wherein one or more of the arms of a star-shaped polymer such as those taught in U.S. Pat. Nos. 4,116,917 and 4,156,673 is, effectively, replaced with an arm prepared by polymerizing a nitrogen containing monomer. Star-shaped polymers wherein all of the arms are homopolymers of isoprene which star-shaped polymers are within the scope of the teaching of both U.S. Pat. Nos. 4,116,917 and 4,156,673 are commercially available and commonly used as VI improvers.
As is well known in the prior art, thickening efficiency of a polymeric additive has, heretofore, generally been the principal, and sometimes the sole consideration in its selection for use as a VI improver. Particularly, polymeric additives which significantly increase the high temperature kinematic viscosity without significantly increasing the low temperature kinematic viscosity are sought. Emphasis was, then, given to obtaining a good VI improver based upon previously specified temperatures and little consideration has been given to the viscosity at even higher temperatures such as 150.degree. C. Now, however, with the advent of the smaller more compact engines which have higher operating temperatures, it is necessary to consider viscosities at these higher temperatures in selecting VI improvers.
Recently, it has been discovered that certain hydrogenated, asymmetrical radial polymers yield particularly effective VI improvers and offer improved high temperature kinematic viscosities. This discovery is described, inter alia, in U.S. Pat. No. 4,849,481. These hydrogenated asymmetric radial polymers comprise a plurality of arms, a portion of which arms are polymers of one or more hydrogenated conjugated diolefins and another portion of which arms are block copolymers comprising a polymeric block containing predominantly one or more monoalkenyl aromatic hydrocarbon monomer units and a polymeric block containing predominantly one or more hydrogenated conjugated diolefin monomer units. These asymmetric radial polymers are prepared by first coupling a suitable blend of living polymers in the same manner used, generally, to prepare star-shaped polymers and then selectively hydrogenating the conjugated diolefin portions of the radial polymers. As indicated in this patent application, these polymers do offer enhanced kinematic viscosities at higher temperatures such as 150.degree. C. and it is anticipated that their use in engine oil compositions for the smaller engines would offer significantly improved performance. The need remains, however, for even further improvement in the high temperature kinematic viscosities and for perhaps even an easier method for obtaining such results.